How is the release of the various chemical messengers controlled?

The activity of the autonomic nervous system is controlled by the hypothalamus which receives messages from many other areas of the brain. One such area, called the cerebral cortex, evaluates the information from the senses and decides what response to make; for example, the decision may be: â˜I can cope with this situation’. Another part of the brain called the limbic system adds emotion, such as fear or anger, to the response decided by the cortex. The type of emotion evoked depends on how the individual feels about the situation. So the limbic system colours or sets the intensity of the response decided by the cortex, for example: â˜I feel angry about this situation’ or â˜I am scared of this situation’.

The final outcome of the interaction of impulses between the cortex and the limbic system is transmitted to the hypothalamus. The hypothalamus translates the decision into a course of action by the body. It organizes nerve impulses to the body organs via the sympathetic and parasympathetic nervous systems, and hormone release into the blood via the pituitary gland.

â˜Get set’ instructions and coping

As we have explained, activation of the stress response results mainly from the pattern of instruction organized by the hypothalamus and sent via two different systems:

â¢ sympathetic nervous system and adrenal medulla (inner part of adrenal gland)

and

â¢ pituitary gland and adrenal cortex (outer part of adrenal gland).

This is summarized in Figure 8.

We have seen that the expression of the stress response, or the way in which we deal with stress, has its roots in fight, flight or resistance. We describe below the physical effects of the three responses.

Get set to fight

When the decision is made by the brain to stay and fight, or when a sustained effort is needed to achieve control over a situation, the hypothalamus signals a mainly noradrenaline release via the sympathetic nervous system. It ‘ is the predominance of noradrenaline that prepares the body organs for fighting. Aggression, anger and hostility are the emotional hallmarks of fighting behaviour. This may first take the form of a sham attack aimed at frightening off the attacker. In animals such as the cat this may involve baring the teeth, hissing, showing claws, arching the back and adopting a threatening posture with fur erect. In humans we often see similar behaviours – making threatening noises and gestures, standing erect, showing and clenching the teeth. A sham attack can be an effective defence mechanism in itself since the attacker may call off his aggression or hesitate, think twice before attacking and give the defender the opportunity to strike first. If the attack is for real, the aim is to achieve control over and dominance of the situation. The body resources must be mobilized to provide maximal mental alertness and body strength. The stronger and more cunning of the pair of fighters will gain control and survive.

Get set to run away

On the other hand, if there is fear or uncertainty about how things will turn out, or doubts about ability to take control, a decision to run away may be made. The hypothalamus signals a predominantly adrenaline secretion which prepares the body for a fast get-away by increasing heart rate and making plenty of energy available for muscular activity.

Both noradrenaline and adrenaline are needed for fighting and fleeing. However, it is the emotion involved which determines the predominance of either noradrenaline or adrenaline, and thus the action appropriate for either running or fighting. This has given rise to a great deal of confusion over the difference between noradrenaline and adrenaline. Most people say, â˜I can feel the adrenaline flowing’ when they are stimulated and excited. In fact, it is noradrenaline, not adrenaline, which is associated with arousal and gives rise to feelings of excitement and drive as well as physical strength. For this reason noradrenaline has been named the â˜kick’ or high performance hormone which in large amounts stimulates special areas in the brain that produce a feeling of pleasure. In contrast, the feelings and sensations associated with high levels of adrenaline are not pleasant. Just think how you felt in the dentist’s waiting room or waiting for a surgical operation: that was adrenaline at work, the predominant hormone released for the fight response – you probably felt like running away.

Get set to resist

In situations where demands persist, activity of the pituitary -adrenal cortex system is predominant. Here cortisol is important in keeping up the supply of energy needed by the body for the effort required in the face of long-term demands.

Maintaining the stability of relationships is a long-term demand and this involves the sex hormone oestrogen and the androgens (of which testosterone is the main hormone). Testosterone is generally regarded as the male sex hormone, secreted by the testes. It also plays a role in women through a closely-related hormone called androstenedione which is released by the adrenal gland.

The sex hormones play an important role in our social behaviour; in forming relationships, bonding between the sexes and in mating. It has been shown that sex hormone levels in the body depend on how secure we feel and, for testosterone, how much dominance and control we feel we have in challenging situations. In situations where we feel helpless and a failure, levels of testosterone in the blood fall. This leads to poor motivation to do things and reduced sexual drive in men. High levels are released when we feel elated, secure and loved. This heightens sexual drive and motivates us into action.

Go

Dealing with the demand: when faced with a demand, the sympathetic nervous system initiates the immediate response mainly through the action of noradrenaline. At the same time, it stimulates the adrenal medulla to release its adrenaline and noradrenaline in order to supplement, sustain and prolong the response. These actions are geared mainly to provide energy for muscular activity (which increases alertness and strength) and to protect the body from blood loss in the event of injury (see Figure 9, page 53).

The first objective of the alarm response is to make available adequate oxygen and fuel for conversion into energy to power the muscle action. Extra oxygen needs to be taken from the air by the lungs and then carried by the blood to the exercising muscles. Sugar and fat stores must be mobilized to provide the fuel for energy production.

Getting the right amount of oxygen and fuel to the muscles

The heart beats faster and harder to circulate larger amounts of oxygen and fuel-rich blood to the muscles. The blood vessels supplying the active muscles become wider (dilate) to increase blood flow to its required destination.

In attempting to supply the muscles with the huge amount of oxygen and fuel-rich blood needed for emergency action, the body has a major problem to overcome. There is only a certain amount of blood in the circulation, so if more blood is needed by the muscles then there will be less available for other body organs. Some vital organs, notably the brain, heart muscle and lungs, must always receive a relatively large blood supply and usually this needs to be increased when supporting muscular work. Other organs are less vital in an emergency. The digestive system, kidneys and skin markedly reduce their activity during periods of stress. This means that their blood supply can be reduced, so the blood vessels supplying these areas decrease in diameter and become narrower (constrict). In other words, to supply the body muscles with adequate oxygen and fuels there must be a redistribution of blood throughout the body: take from the non-vital areas and give to the vital.

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